The present invention relates generally to an organic LED device, and more specifically to a top emission organic LED device suitable for significant screen size.
Organic LEDs are well known today. When used in a planar display device, they can be driven by an active matrix drive method such as previously used for a liquid crystal display device. The active matrix deive can be used for a top emission structure or a bottom emission structure. FIG. 9 is a cross-sectional view of an organic LED device using the top emission structure according to the Prior Art. The organic LED device shown in FIG. 9 comprises a thin film transistor (TFT) structure 82 formed of p-type doped polycrystalline silicon (poly-Si) on a glass substrate. The TFT structure 82 is insulated from an upper structure by an insulating film 84. A reflective metal anode 86 (such as molybdenum (Mo), nickel (Ni) and platinum (Pt)) is formed on an upper portion of the insulating film 84. A hole injection layer 88 is formed on an adjacent and upper layer of the reflective anode 86. A hole transport layer 90 and an electron transport layer 92 are formed on an upper layer of the hole injection layer. A translucent cathode 94 is formed on an upper layer of the electron transport layer 92. This cathode 94 transmits a light beam generated by the organic LED therethrough and also supplies electrons. For example, the cathode 94 can be formed of a material having a small work function, such as aluminum (Al), sodium (Na), calcium (Ca), magnesium-silver (MgAg), barium (Ba) and strontium (Sr). A buffer layer 96 and a glass protective layer 98 are formed on the cathode 94. Thus, the top emission structure is formed.
The top emission type organic LED device shown in FIG. 9 is more efficient than the bottom emission type in that an aperture ratio can be improved without depending on the dimension of the TFT. However, the top emission type requires the very thin cathode 94 (about 10 nm) film in order to impart a transparency thereto. Therefore, the cathode 94 has has a disadvantage of being inevitably high in resistance. Because cathode 94 is high in resistance there is significant deop in cathode voltage. This increases from an end portion of a screen to a center portion thereof. Therefore, as the area of the organic LED device becomes larger, it is difficult to apply a sufficient voltage for driving the TFT from the end portion of the screen to the center portion of the screen. In order to reduce the voltage drop through the above-described cathode 94, it is possible to add a low-resistance layer such as ITO, IZO, SnOx, and InOx on the cathode 94. Nevertheless, the ITO has some resistance. Therefore, when a large screen, for example ten inches, uses top emission organic LEDs it is difficult to provide an even level of intensity across the screen.
FIG. 10 shows a Prior Art driver circuit 100 of a cathode-common mode, which uses a p-type driver TFT 102 and is used for driving the top emission organic LEDs. A drain electrode 102d of the driver TFT 102 is connected to an organic LED element 104, a source electrode 102s is set at a common potential, and the driver circuit 100 is driven in the cathode-common mode. A gate electrode 102g of the driver TFT is connected to a switching TFT 108 to permit selective driving of the organic LED element 104. The Ids current between the source and drain of the driver TFT 102 in a saturation region thereof is approximately proportional to (Vgsxe2x88x92Vth)2 in the top emission structure shown in FIG. 10. xe2x80x9cVgsxe2x80x9d is a voltage between the gate and the source, and xe2x80x9cVthxe2x80x9d is a threshold voltage. Because Ids is given by a function only of the Vgs in the conventional top emission structure, the cathode-common mode is adopted. Variation of the Vgs of the TFT is accomodated by characteristic variation of the organic LED.
The following Table 1 lists the types of TFTs that can be used for preventing the change of the Vgs following the characteristic variation of the organic LED. In Table 1, a reference symbol xe2x80x9ccirclexe2x80x9d denotes types that can accomodate the characteristic variation of the organic LED element, and a reference symbol xe2x80x9ccrossxe2x80x9d denotes types that are not capable of accomodating the characteristic variation of the organic LED element.
Even if any of the n-type TFT or the p-type TFT are used, the characteristic variation of the organic LED element can be accomodated by any of the anode-common mode and the cathode-common mode, respectively, when consideration is made only for that characteristic variation as described above. However, another disadvantage (as described below) will occur in the case of forming an anode-common structure by use of the n-type TFT as the driver TFT.
FIG. 11 shows a cross-sectional structure of the driver circuit of FIG. 10 where the anode-common structure is formed by the n-type driver TFT 102. The pixel also comprises switching TFT 108, an anode 110, a cathode 106 and LED element 104. In the conventional top emission structure, the resistive cathode cannot be arranged as a lower electrode because the injection efficiency and light emission efficiency are significantly lowered. Therefore, in the case of forming the top emission structure by adopting the anode-common structure using the n-type TFT, as shown in FIG. 11, it becomes necessary to form a contact hole for anode 110 and cathode 106 in each pixel. This lowers the aperture ratio in the pixel of the organic LED element 108 which is undesirable. Such contact holes are not efficienct or productive and add to the cost. On the other hand, the cathode-common mode using the n-type TFT cannot restrict the variation of the Vgs following the characteristic variation of the organic LED and is inferior in display characteristics.
Accordingly, an object of the present invention is to provide a top emission organic LED device with a less expensive construction than prior art devices.
Another object of the present invention is to provide a top emission organic LED device for a wide screen.
Another object of the present invention is to provide a to emission organic LED device of the foregoing type which has a high aperature ration.
The invention resides in an organic LED device comprising a substrate, a first driver TFT on the substrate, a second driver TFT on said substrate, and an insulating film on the substrate, the first driver TFT and the second driver TFT. There is a common anode on the insulating film. A first organic LED element is on a first portion of the anode and configured as a top emission struction, and a second organic LED element is on a second portion of the anode and configured as a top emission structure. A first cathode extends into the insulating film and electrically connects the first LED element with the first driver TFT. A second cathode extends into the insulating film and electrically connects the second LED element with the second driver TFT.
There are other features of the present invention. For example, N-type driver TFTs are used in the top emission structure. It was found that by adopting the anode-common structure in the organic LED device using the n-type driver TFT, an influence of the characteristic variation of the organic LED element to the Vgs can be minimized, and the characteristics can be stabilized. Also, the anode is planar and formed of a low-resistance material such as Al, Ni and Co. By use of this type of anode the common electrode connected to the plurality of pixels is lowered in resistance, thus making it possible to provide the organic LED device of a large area. The anode is formed as lines or a plane, thus making it possible to use the anode as the common electrode. The common anode configuration simplifies the manufacturing process.
It is preferable that the driver TFT include any of n-type amorphous silicon and n-type polycrystalline silicon as an active layer. It is also preferable that the organic LED device include at least a light emitting portion and an electron transport portion, a part of each being formed self-consistently.